Radiotherapy apparatus employ a source of ionizing radiation that is directed towards a cancerous region of a patient. Harm is caused to the cancerous tissue, thereby moving the patient towards cure.
The radiation is equally apt to cause harm to healthy tissue, and therefore a number of steps are taken to limit the exposure of non-cancerous tissue to radiation, and to enhance the exposure of cancerous tissue. Collimators and treatment plans seek to direct the radiation to the target areas, to limit exposure in other areas, and (where necessary) to balance these two objectives satisfactorily.
As the systems become more complex, the opportunity for error as a result of system malfunction becomes in principle greater, and therefore additional layers of checking and control circuitry are added to make this in fact unlikely. These additional layers are themselves theoretically at risk of malfunction and therefore the system is designed to fail safe, i.e. to cease the treatment if anomalies are detected.
One such check is directed at the recent average power output of the radiation source. If this power output were to deviate exceptionally from the expected level, then this is generally regarded as an indication that some otherwise undetected malfunction has occurred with the radiation source and that might cause the distribution of the dose to the tumor and/or the healthy tissue to be different from the planned distribution. Accordingly, the recent average power output of the source is checked and the treatment stopped if this falls outside acceptable limits.